This text should not be autosized (its computed font-size should remain 16px), as autosizing usually causes text to wrap onto more lines, which might cause it to overflow the block's max-height (the float:left doesn't affect this).
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.

This text should be autosized to 40px computed font-size, since the parent is float:left so the height is considered to be unconstrained (ancestors of the parent are ignored).
Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.